One critical procedure in the manufacture of computer keyboards is decorating. This is the application of letters and numbers on the keyboard. The graphics cannot wear off during the useful life of the keyboard. This is a challenging objective because of normal abrasion from finger typing and chemical attack, such as from hand creams.
The undisputed best method for decorating keyboards is dye-sublimation. This process involves impregnating the plastic with a dye to form the character. Thus, the plastic must be worn away before the character will wear. However, dye-sublimation can only be used to place darker characters on a lighter substrate, such as black characters on white keys because a white sublimation dye does not exist. This is not a problem for standard keyboards, but notebook computer keyboards are frequently black with white characters. Also, some designs of desktop keyboards call for black keys with white characters.
The most popular and economic technique of decorating black keyboards is pad printing. However, the inks used in pad printing are typically not abrasion or chemical resistant. Therefore, most manufacturers apply a protective clear coating of polyurethane to protect the graphics from wear and chemical attack. This protective coating is expensive and difficult to apply. Most pad printing inks are made from thermoplastic materials like polyvinylchloride, which are dissolved in a solvent. Since such materials are not reactive, they have infinite pot life as long as the operator continues to add solvent. But these materials are inherently poor against chemical attack and abrasion, and because they are not reactive, they often do not adhere to the substrate well. They are mainly chosen for their pot life and quick drying capabilities and were never designed for a high wear environment like the keyboard application. Still, there are many commercially available reactive epoxy and polyurethane pad printing inks which have better chemical and abrasion resistance than the thermoplastic inks, but are still not as good as the coated indicia. This invention involves the creation of a polyurethane based pad printing ink which is far superior to any other commercially available pad printing ink and which is better than or equal to many protective clear coatings. The formulation used is similar to that used by automobile companies for the protective overcoat of automobiles. Hexamethylene diisocyanate/polyester polyurethanes, as are used in this invention, are considered to be the most abrasion resistant class of polyurethane. This invention effectively uses the protective clearcoat polymer and incorporates it into the pad print ink, thus eliminating the need for a protective clearcoat over the characters of a keyboard.
U.S. Pat. No. 4,776,270 to Kumamoto is to a reactive ink on a keyboard key with no overcoat. However, the reaction is driven by ultraviolet light and the material is not a urethane. U.S. Pat. No. 4,341,689 to Doshi et al is to a reactive urethane system, which mentions the inclusion of pigments, but not in the context of keyboard printing. U.S. Pat. No. 4,980,221 to Kobayashi et al and U.S. Pat. No. 4,903,601 to Fukui are to keyboard printing with urethane inks, but not reactive inks.